This invention relates to ink jet printing systems and similar drop marking systems in which a supply of electrically conductive ink is provided to a nozzle. The ink is forced through a nozzle orifice while at the same time an exciting voltage is applied to the nozzle to cause the stream of ink to break into droplets which can be charged and deflected onto a substrate to be marked. Such ink jet technology is well known and, for example, see U.S. Pat. Nos. 4,727,379 and 4,555,712.
To ensure proper operating conditions for consistent printing quality, the exciting energy or voltage applied to the nozzle must be properly set during operation of the system. Presently, most ink jet printers require manual setting of the energy applied to the ink stream as it exits the nozzle. The appropriate value is either empirically determined by comparing what is seen to an existing diagram or by determining the drop separation point and comparing it with machine specifications. In either case, the resulting print quality varies.
Efforts to provide automatic control of the modulation voltage have concentrated on detecting separation point position, relative to a fixed location, such as the charge tunnel. See, for example, published European patent specification EPA 0287373. Another approach is disclosed in U.S. Pat. No. 4,638,325 which utilizes a small charging electrode and a downstream electrometer by which the drop separation point can be determined by observing the current at the electrometer as the separation point approaches the small electrode. In the '325 patent, the maximum current is produced when drop separation is closest to the small charging electrode.
The above method does not take into account the basic reason for maintaining consistent drop charging conditions. The drop separation point varies greatly with the surface tension and viscosity of the ink, therefore, simply holding the separation point constant still results in different satellite conditions and variable print quality. In short, maintaining the drop separation point constant is not a satisfactory solution to the problem.
What is desired is a system which can determine a range of proper printing nozzle drive voltages and then compute a satisfactory intermediate value within said range. Such a system should be temperature independent over a wide range of operating temperatures to result in a significantly better control system.
It is accordingly an object of the present invention to provide such a nozzle drive control system which improves upon known techniques.
It is a further object of the invention to provide a nozzle control system which can accurately monitor the condition of the satellite drops and the drop breakoff point and compute therefrom a satisfactory range of nozzle drive voltages for operating an ink jet printer.
A further advantage of this invention is that it allows automation of the nozzle voltage for best quality printing using a continuous ink jet printer regardless of ink type and temperature. This invention avoids problems with recombining satellites that occur when holding the drop separation point constant while ink type and temperature vary. These cause unwanted charge variations because a satellite which carries part of the charge of its parent charged drop will transfer that charge to the drop following when merging occurs. These and other objects of the invention will be apparent from the remaining portion of this specification.